Alternatively powered vehicles include the hybrid electric vehicle (HEV) and battery electric vehicle (BEV). Both the series hybrid and BEV have 100% of propulsion energy coming from electricity. The series hybrid uses an internal combustion engine (ICE) to power a generator that produces electricity. The parallel hybrid powers the vehicle by a mechanical combination of electric motors and an ICE. In all cases, the drivetrain needs an electric motor, a traction battery and an auxiliary method of obtaining electricity. These auxiliary power units (APUs) are typically a downsized, highly efficient ICE or fuel cell for a zero emissions alternative.
Alternatively powered vehicles also have a powertrain subsystem that allows for the recuperation of vehicle kinetic energy during braking events, such as decelerating or maintaining speed when going downhill, and for reuse during a future torque demanding event, such as accelerating, maintaining speed going uphill, or powering auxiliary loads.